This invention relates to the treatment of aluminum articles to protect them against corrosion. In a particular sense, it is directed to a process for applying a coating of zinc on surfaces of an aluminum article, such coating being suitable for forming a zincdiffused surface layer to protect the article against corrosion. The term "aluminum" as used herein refers to aluminum metal and alloys thereof.
It is already known to protect an aluminum article against corrosion by depositing a coating of zinc on surfaces of the aluminum article, as by a so-called zincating treatment, and then heating the article to a temperature at which the zinc diffuses into the coated surfaces and surface-adjacent regions of the article. The depth to which the zinc diffuses and the concentration of zinc in the diffused surface layer of the aluminum article are dependent to a considerable extent on the amount of zinc deposited. Since the zinc acts as a sacrificial anode for the protection of the aluminum, the effectiveness of the corrosion protection provided by the zinc (including the length of time such protection is effective) is to a considerable extent dependent both on the thickness of the zinc-diffused surface layer and the concentration of Zn in such layer.
In order to achieve satisfactory protection, it is necessary that the aluminum surfaces be protected all over and that the Zn-diffused layer should be free from localized areas in which diffused Zn is absent or present only in insufficient quantity. It is therefore important that the initially deposited zinc should be firmly attached to the underlying aluminum and free from localized blemishes.
In a conventional zincating treatment in a bath containing 300-500 g/1 NaOH and 50-100 g/1 dissolved ZnO at a temperature of 20.degree.-30.degree. C., the amount of zinc deposited is normally about 1.0 g/m.sup.2, although this is dependent upon the duration of the immersion in the bath and the conditions of the pretreatment of the aluminum surface before dipping.
In tests, in which the temperature of the bath was raised to 40.degree.-60.degree. C. to increase the rate of zinc deposition, abnormal deposition appeared; that is, zinc deposited locally with irregular thickness and moreover bonding was very poor and the zinc layer was not suitable for zinc diffusion treatment. On the other hand, when the bath temperature was lowered to 20.degree.-30.degree. C. to suppress abnormal deposition, zinc initially deposited rapidly but then the rate of deposition greatly decreased and more than 20 minutes was required to deposit zinc to the fullest extent of about 10-15 g/m.sup.2. Further, it was very difficult to reproduce the results and therefore the method was considered unsuitable for industrial use.
The problems resulting from slow deposition at temperatures normally employed in a single stage zincating treatment can, however, be alleviated to a considerable extent by employing the two-stage zincating treatment described in the copending U.S. patent application of Masamichi Suzuki (one of the applicants herein), Atsushi Sugihara, Tadaaki Sano (one of the applicants herein), and Toshihiro Suzuki (one of the applicants herein), Ser. No. 06/491,022, filed May 3, 1983, for "Process for Coating Aluminum with Zinc," and assigned to the same assignee as the present application.
On the other hand, that two-stage process does not overcome other problems encountered in the industrial application of any zincating process, which depends upon the use of a zincating bath having a high concentration of sodium hydroxide. Because of its high NaOH content, for example 350 g/l, the known bath is relatively viscous and is therefore difficult to employ successfully in the treatment of articles or assemblies of complicated form, particularly where there are narrow recesses, such as may occur, for example, in evaporators, condensers, and automobile heat exchangers constituted of flattened tubes and fins. If an attempt is made to treat these articles in the known zincate baths mentioned above, the viscous zincate bath liquor does not penetrate rapidly or even completely into all the recesses,with the result that the deposition of zinc at such localities may be incomplete or, at worst, nonexistent. This will be readily understood by considering an assembly made up of flattened tubes in serpentine form with corrugated fins fixed to them by pinching.
In addition to the problem of possibly inadequate penetration into recesses and consequently inadequate deposition of zinc, problems also arise in removing excess bath liquor from the treated assembly. Because of the high viscosity of the zincate bath liquor, it drains poorly from the surface of the treated assembly and, particularly, it tends to be retained within recesses. Consequently, there is not only a high loss of the dissolved solids content of the bath, carried over with the dipped assemblies, but also large quantities of water are required to wash off these residues from the assemblies. Since the wash water contains dissolved sodium hydroxide, costly waste water treatment equipment must be employed before the wash water can be discharged to waste.
It would be desirable to overcome the foregoing disadvantages, and, in particular, to achieve a rapid and uniform deposition of an adherent zinc layer on surfaces of an aluminum article by dipping in an aqueous bath having a low content of dissolved solids and consequently low viscosity.